Aero-static gas bearings use a thin layer of pressurized gas between surfaces to allow relative movement with virtually no friction. This layer of gas is typically only 5 to 25 microns deep. The area of the pad is sized to keep the surfaces separated under their weight and external loads when pressurized. The resulting bearing stiffness is proportional to the energizing pressure supplied to the bearing pad and inversely proportional to the square of gas film depth between the surfaces. A variety of methods have been utilized to evenly distribute the lifting pressure across the pad by utilizing a pattern of small orifices, porous media, and various groove geometries machined into the pad surfaces. Some of these are described in the following patents and all impact the resulting damping that may be realized: Saulgot et al, U.S. Pat. No. 3,698,774; Ida, et al., U.S. Pat. No. 4,392,751; Klein et al., U.S. Pat. No. 4,521,121; Stauber, U.S. Pat. No. 4,558,909; Fleury et al., U.S. Pat. No. 6,505,698
Damping of gas bearings has typically been derived from the inherent force created from squeezing the viscous gas between the small gaps between the bearing pad and surface of the supported member or by adding coulomb friction, which adds to bearing wear and drag. Although resulting damping coefficients are better than for purely mechanical bearings without friction or liquid dampers, damping ratios as a result of squeeze film damping are typically well under 10% and decrease inversely proportional to the cube of the gap or the amplitude of the relative motion. This damping is nearly ineffective for gaps greater than 25 microns, where it is often required to operate in high speed rotating machinery design. The result is resonances or instability in the air bearing suspension system.
The squeeze film damping also changes with gas temperature and associated change in gas viscosity. Various techniques have been used to adjust the gaps to control damping ratios, but maximum achievable damping is still limited. Applications requiring more damping, such as hard drives, utilize active control techniques with electronic position sensing, piezo-actuator adjustment of gaps, and digital electronic control. However, these techniques are complicated and are not robust enough for industrial automation or high speed rotating machinery applications. Foil bearings rely on coulomb friction from support springs to provide limited damping, but the resulting damping decreases for increasing excursion amplitudes.